Hydraulic Jet Pump and Method for Use of Same

ABSTRACT

A hydraulic jet pump for transference of a fluid medium and method for use of the same are disclosed. In one embodiment of the hydraulic jet pump, a power fluid inlet adapter communicates through a jet nozzle to a mixing tube along a power fluid inlet flow path. In a first configuration, an axial diverter member traverses the jet nozzle and the diffusing chamber. In a second configuration, the mixing tube has an inlet that is axially aligned with the jet nozzle. In a third configuration, the mixing tube has at least two inlets, one that is axially aligned with the jet nozzle and another that is angularly offset from the alignment of the jet nozzle and the mixing tube. One of the three configurations is selected prior to downhole deployment of hydraulic jet pump.

PRIORITY STATEMENT & CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority from co-pending (1) U.S. Provisional Patent Application No. 63/052,666 entitled “Hydraulic Jet Pump and Method for Use of Same” and filed on Jul. 16, 2020 in the names of Laslo Olah et al.; and (2) U.S. Provisional Patent Application No. 62/929,596 entitled “Jet Pump” and filed on Nov. 1, 2019 in the names of Laslo Olah et al.; both of which are hereby incorporated by reference, in entirety, for all purposes.

TECHNICAL FIELD OF THE INVENTION

This invention relates, in general, to hydraulic jet pumps and, in particular, to hydraulic jet pumps for the removal of fluid mediums with low viscosity, such as water or light crude oil, during hydrocarbon production from a well, for example.

BACKGROUND OF THE INVENTION

Without limiting the scope of the present invention, the background will be described in relation to hydrocarbon producing wells where one or more extreme conditions—such as great depths, extreme deviations, high concentrations of gas and sand, and heavy, corrosive fluid—may occur and encumber production. In a healthy, optimally producing well without encumbered production conditions, high pressure hydrocarbon or oil flow has the ability to lift this liquid to the surface. Under more extreme conditions, however, flow conditions may degrade. Under such extreme conditions, artificial-lift techniques using equipment like submergible pumps may be insufficient as the equipment may stick, lock, or wear down. Existing hydraulic jet pumps with no moving parts, however, may not be sufficiently versatile to operate and create lift across a full range of conditions. Accordingly, there is a need for improved hydraulic jet pumps and methods for use of the same that efficiently operate across a full range of conditions, including extreme conditions, such as great depths, extreme deviations, high concentrations of gas and sand, and heavy, corrosive fluid.

SUMMARY OF THE INVENTION

It would be advantageous to achieve a hydraulic jet pump and method for use of same that would improve upon existing limitations in functionality. It would also be desirable to enable a mechanical-based solution that would provide enhanced operational across a full range of conditions, including extreme conditions, such as great depths, extreme deviations, high concentrations of gas and sand, and heavy, corrosive fluid. To better address one or more of these concerns, a hydraulic jet pump for transference of a fluid medium and method for use of the same are disclosed. In one aspect, some embodiments of the hydraulic jet pump include a power fluid inlet adapter that communicates through a jet nozzle to a mixing tube along a power fluid inlet flow path. In a first configuration, an axial diverter member traverses the jet nozzle and the diffusing chamber. In a second configuration, the mixing tube has an inlet that is axially aligned with the jet nozzle. In a third configuration, the mixing tube has at least two inlets, one that is axially aligned with the jet nozzle and another that is angularly offset from the alignment of the jet nozzle and the mixing tube. One of the three configurations is selected prior to downhole deployment of the hydraulic jet pump.

In another aspect, some embodiments of a method for use of a hydraulic jet pump include providing the hydraulic jet pump of the type having three configurations as previously described, for example. The methodology also includes selecting one of the three configurations prior to downhole deployment of the hydraulic jet pump. The selection may include analyzing at least one of wellhead pressure, target produced flow rate, maximum velocity, cavitation pressure, geometric dimensions, pump metrics, and power fluid pressure and flow rate, for example. These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the features and advantages of the present invention, reference is now made to the detailed description of the invention along with the accompanying figures in which corresponding numerals in the different figures refer to corresponding parts and in which:

FIG. 1 is a schematic illustration depicting one embodiment of an onshore hydrocarbon production operation employing a hydraulic jet pump, according to the teachings presented herein;

FIG. 2 is a schematic illustration depicting one embodiment of the onshore hydrocarbon production operation of FIG. 1 in a first stage of removing a fluid medium by utilizing the hydraulic jet pump;

FIG. 3 is a schematic illustration depicting one embodiment of the onshore hydrocarbon production operation of FIG. 1 in a second stage of removing the fluid medium by utilizing the hydraulic jet pump;

FIG. 4 is a schematic diagram depicting one embodiment of the hydraulic jet pump of FIG. 1 in a first configuration;

FIG. 5 is a schematic diagram depicting one embodiment of the hydraulic jet pump of FIG. 1 in a second configuration;

FIG. 6 is a schematic diagram depicting one embodiment of the hydraulic jet pump of FIG. 1 in a third configuration;

FIG. 7 is another schematic diagram depicting the hydraulic jet pump of FIG. 6 in additional detail;

FIG. 8 is a flow chart depicting one embodiment of a method for transference of a fluid medium utilizing the hydraulic jet pump of FIG. 1, according to the teachings presented herein; and

FIG. 9 is a flow chart depicting another embodiment of a method for transference of a fluid medium utilizing the hydraulic jet pump of FIG. 1, according to the teachings presented herein.

DETAILED DESCRIPTION OF THE INVENTION

While the making and using of various embodiments of the present invention are discussed in detail below, it should be appreciated that the present invention provides many applicable inventive concepts, which can be embodied in a wide variety of specific contexts. The specific embodiments discussed herein are merely illustrative of specific ways to make and use the invention, and do not delimit the scope of the present invention.

Referring initially to FIG. 1, therein is depicted one embodiment of a hydraulic jet pump 10 being employed in an onshore hydrocarbon production operation 12, which may be producing oil, gas, or a combination thereof, for example. A wellhead 14 is positioned over a subterranean hydrocarbon formation 16, which is located below a surface 18. A wellbore 20 extends through the various earth strata including the subterranean hydrocarbon formation 16. A casing string 24 lines the wellbore 20 and the casing string 24 is cemented into place with cement 26. Perforations 28 provide fluid communication from the subterranean hydrocarbon formation 16 to the interior of the wellbore 20. A packer 22 provides a fluid seal between production tubing 30 and the casing string 24. Composite coiled tubing 34, which is a type of production tubing 30, runs from the surface 18, wherein various surface equipment 36 are located, to a fluid accumulation zone 38 containing a fluid medium F, such as hydrocarbons like oil or gas, fracture fluids, water, or a combination thereof. As shown, the hydraulic jet pump 10 is coupled to a lower end 40 of the composite coiled tubing 34.

Referring now to FIG. 2 and FIG. 3, as shown, the hydraulic jet pump 10 is positioned in the fluid accumulation zone 38 defined by the casing string 24 cemented by the cement 26 within the wellbore 20. The hydraulic jet pump 10 is incorporated into a downhole tool 50 connected to the lower end 40 of the composite coiled tubing 34 and, more particularly, the hydraulic jet pump 10 includes a housing 52 coupled by a coupling unit 54 to the composite coiled tubing 34. At the other end of the housing 52, a coupling unit 56 couples the hydraulic jet pump 10 to a valve assembly 58 having a standing valve and ports 60 therein, which provide fluid communication to the downhole tool 50. It should be appreciated that a variety of downhole tool-configurations may be employed depending on the particular application that the hydraulic jet pump 10 is assigned.

In operation, as will be discussed in further detail hereinbelow, one of three configurations may be selected for the hydraulic jet pump 10. Then, to begin the processes of transferring the fluid medium F, the hydraulic jet pump 10 is positioned in the fluid accumulation zone 38. Initially, as shown best in FIG. 2, the hydraulic jet pump 10 is completely submerged in the fluid medium F, which, as mentioned, may include hydrocarbons such as oil and/or gas, fracture fluid, water, or combinations thereof. The hydraulic jet pump 10 is actuated and selective operation of the hydraulic jet pump 10 begins. As time progresses, as shown best in FIG. 3, the hydraulic jet pump 10 pumps the fluid medium F, which may be a production fluid or a production inhibiting fluid, for example, to the surface 18. The process of pumping the fluid medium F continues until the hydraulic jet pump 10 is stopped.

Referring now to FIG. 4, one embodiment of the hydraulic jet pump 10 in a first configuration Ci is depicted. The housing 52 houses a power fluid inlet adapter 70, a jet nozzle 72, and a mixing tube 74 positioned therein. As shown, the power fluid inlet adapter 70, the jet nozzle 72 and the mixing tube 74 have a longitudinal axis L therethrough. The power fluid inlet adapter 70 communicates through the jet nozzle 72 to the mixing tube 74 along a power fluid inlet flow path 76. An annulus 78 is formed between the housing 52 and the power fluid inlet adapter 70, the jet nozzle 72, and the mixing tube 74. A flow chamber 80 having a surface flow path 82 is provided in the annulus 78. A sealing element 84 seals the annulus 78 at a lower end of the hydraulic jet pump 10.

The jet nozzle 72 has a fluid inlet 90, an area of contraction 92, and an exit channel 94. The exit channel 94 includes an exit diameter d₁. The mixing tube 74 includes a throat 96 transitioning to a diffusing chamber 98. The throat 96 may have an inlet 100 that is axially aligned with the exit channel 94 along the longitudinal axis L. Also, the diffusing chamber 98 includes an outlet 102 opposite the throat 96.

In the illustrated first configuration C₁, an axial diverter member 110 traverses the longitudinal axis L from the exit channel 94 to the throat 96. The axial diverter member 110 has an upper end 112 and a lower end 114 with the upper end 112 having a diameter d₂ smaller than the exit diameter d₁. In one embodiment, the axial diverter member 110 includes a gondola shape. In another embodiment, the axial diverter member includes an elliptical nose cone at the upper end. In still another embodiment, the axial diverter member includes a conical shape at the lower end.

In one operational embodiment of a lifting system or methodology utilizing the hydraulic jet pump 10 having the first configuration C₁, the hydraulic jet pump 10 is run downhole into the wellbore 20 and positioned at the desired location. At the surface 18, a multiplex pump (not shown) or other device may be utilized to pressurize and inject power fluid into the wellbore 20. The fluid travels downhole through the wellbore 20 to the hydraulic jet pump 10 through the power fluid inlet flow path 76. At the jet nozzle 72, the fluid enters the fluid inlet 90 and fluid pressure is reduced using the Venturi effect as the fluid travels through the area of contraction 92 to the exit channel 94 and into the throat 96 of the mixing tube 74. The fluid travels around the axial diverter member 110, which augments the fluid pressure reduction using the Venturi effect. Additionally, when utilizing the axial diverter member 110, the velocity profile of the accelerated power fluid provides for the accelerated flow to become turbulent and the resulting vortices assist with suctioning reservoir fluid into the mixing tube 74. Therefore, within the mixing tube 74, the reduced fluid pressure draws reservoir fluid along reservoir flow path 116 into the diffusing chamber 98, where the fluids combine as shown by arrow 118 and static pressure is increased to raise the fluids to the surface 18 via the surface flow path 82.

Referring now to FIG. 5, one embodiment of the hydraulic jet pump 10 in a second configuration C₂ is depicted. As with the first configuration in FIG. 4, the housing 52 includes the power fluid inlet adapter 70, the jet nozzle 72, and the mixing tube 74 positioned therein. In one operational embodiment of a lifting system or methodology utilizing the hydraulic jet pump 10 having the second configuration C₂, the hydraulic jet pump 10 is run downhole into the wellbore 20 and positioned at the desired location. At the surface 18, a multiplex pump (not shown) or other device may be utilized to pressurize and inject power fluid into the wellbore 20. The fluid travels downhole through the wellbore 20 to the hydraulic jet pump 10 through the power fluid inlet flow path 76. At the jet nozzle 72, the fluid enters the fluid inlet 90 and fluid pressure is reduced using the Venturi effect as the fluid travels through the area of contraction 92 to the exit channel 94 and into the throat 96 of the mixing tube 74. Within the mixing tube 74, the reduced fluid pressure draws reservoir fluid along reservoir flow path 116 into the diffusing chamber 98, where the fluids combine as shown by arrow 118 and static pressure is increased to raise the fluids to the surface 18 via the surface flow path 82.

Referring now to FIG. 6 and FIG. 7, one embodiment of the hydraulic jet pump 10 in a third configuration C₃ is depicted. As with the first configuration C₁ in FIG. 4 and the second configuration C₂ in FIG. 5, the housing 52 includes the power fluid inlet adapter 70, the jet nozzle 72, and the mixing tube 74 positioned therein. In one operational embodiment of a lifting system or methodology utilizing the hydraulic jet pump 10 having the third configuration C₃, the hydraulic jet pump 10 is run downhole into the wellbore 20 and positioned at the desired location. At the surface 18, a multiplex pump (not shown) or other device may be utilized to pressurize and inject power fluid into the wellbore 20. The fluid travels downhole through the wellbore 20 to the hydraulic jet pump 10 through the power fluid inlet flow path 76. At the jet nozzle 72, the fluid enters the fluid inlet 90 and fluid pressure is reduced using the Venturi effect as the fluid travels through the area of contraction 92 to the exit channel 94 and into the throat 96 of the mixing tube 74. At the throat 96, the inlet 100 is axially aligned with the exit channel 94 along the longitudinal axis L. An inlet 130 is also positioned at the throat 96; however, the inlet 130 is angularly offset from the longitudinal axis L. Within the mixing tube 74, the reduced fluid pressure draws reservoir fluid along reservoir flow path 116 into the diffusing chamber 98 via dual flow paths 132, 134, where the fluids combine as shown by arrow 118 and static pressure is increased to raise the fluids to the surface 18 via the surface flow path 82. The use of inlets 100, 130 provides two intake sections which generate a secondary suction effect that increase the suctioned reservoir fluid flow rate.

In each of the first, second, and third configurations C₁, C₂, C₃, the hydraulic jet pump 10 includes minimal moving parts and a compact, durable design, that decreases the risk of equipment failure to ensure efficient operation across a full range of conditions, including extreme conditions, such as great depths, extreme deviations, high concentrations of gas and sand, and heavy, corrosive fluid. Once installed, the hydraulic jet pump 10 requires little to no maintenance. Once the fluid lifting operation is complete, the hydraulic jet pump 10 may be retrieved by reversing the flow of the pressurized fluid.

Referring now to FIG. 8, one embodiment of a method for transference of a fluid medium utilizing a hydraulic jet pump is presented. The methodology begins at block 150 before advancing to block 152 where the wanted depth of the hydraulic jet pump is determined. At decision block 154, if the wanted depth is between about 0 feet and 900 feet, then the methodology advances to block 156 where the hydraulic jet pump of the first configuration is selected. As previously discussed, the axial diverter member creates an amplified Venturi effect compared to the hydraulic jet pump of the second configuration. The presence of the axial diverter member creates jet stream changes in the fluid flow that result in a velocity profile change in the suction zone proximate the throat of the mixing tube. The velocity profile downhole of the jet nozzle is asymmetrical in the first configuration of the hydraulic jet pump as opposite to symmetrical in the second configuration of the hydraulic jet pump. The use of the axial diverter member increases the active surface area of interaction between the power fluid and the reserve fluid, which causes the change in the velocity profile. Following the selection of the first configuration for the hydraulic jet pump, the methodology ends at block 158.

Returning to the decision block 154, if the depth is outside the range of about 0 feet to about 900 feet, then the methodology advances to decision block 160, where if the wanted depth of the hydraulic jet pump is between about 600 feet and about 2700 feet then the methodology advances to block 162, where the third configuration of the hydraulic jet pump is selected with a dual intake design. As previously discussed, the dual intake design optimizes the usage of remaining kinetic energy from the power fluid to suction additional reservoir fluid at a second intake at the throat of the mixing tube. Following the selection of the third configuration for the hydraulic jet pump, the methodology ends at block 158. Returning to decision block 160, if the wanted depth of the hydraulic jet pump is not between about 600 feet and about 2700 feet, then the methodology advances to block 164, where the second configuration of the hydraulic jet pump is selected. Following the selection of the second configuration for the hydraulic jet pump, the methodology ends at block 158.

FIG. 9 is a flow chart depicting another embodiment of a method for transference of a fluid medium utilizing a hydraulic jet pump. The methodology provides for the selection of a hydraulic jet pump configuration; namely, the first configuration C₁, the second configuration C₂, and the third configuration C₃. The methodology begins at block 170 and progresses to block 172 where the operating parameters for the hydraulic jet pump are analyzed at blocks 174, 176, 178. At the block 174, well depth is analyzed. The depth of the well determines the static pressure at the outlet of the hydraulic jet pump, where there is resistance to the pumping of reservoir fluid. For proper operation of the hydraulic jet pump, the pressure within the diffusing chamber must be higher than the pressure at the outlet of the hydraulic jet pump. At block 176, wellhead pressure is analyzed. The higher the presser at the wellhead, the greater the produced flow rate as the pressure differential between the power fluid flow rate and the produced well fluid flow rate is large. At block 178, target produced flow rate is analyzed and expressed as an absolute value or as a ratio, for example, between the power fluid flow rate and the produced well fluid flow rate.

Following the analysis at the blocks 174, 176, 178, the methodology advances to block 180 where constraints are analyzed to ensure proper operation of the hydraulic jet pump.

At block 182, the maximum fluid velocity in the hydraulic jet pump is determined such that subsonic flow is achieved. At block 184, cavitation pressure is analyzed. The lowest status pressure (P_(Static)) should be greater than the cavitation pressure (P_(Cavitation)). The geometric dimensions of the design are analyzed at block 186. The diameter of the throat (D_(Throat)) should be greater than the diameter of the nozzle (D_(Nozzle)) and the maximum diameter of the housing (D_(AssmMax)) should be smaller than the casing inner diameter (D_(Casing)).

After the analysis of the operating parameters and the constraints, the pump parameters are determined through empirical mathematical equations at block 188. In particular, pump shape is determined at block 190, pump metrics determined at block 192, and power fluid pressure and flow rate are determined at block 194. The methodology then advances to block 196 where the methodology concludes.

The order of execution or performance of the methods and techniques illustrated and described herein is not essential, unless otherwise specified. That is, elements of the methods and techniques may be performed in any order, unless otherwise specified, and that the methods may include more or less elements than those disclosed herein. For example, it is contemplated that executing or performing a particular element before, contemporaneously with, or after another element are all possible sequences of execution.

While this invention has been described with reference to illustrative embodiments, this description is not intended to be construed in a limiting sense. Various modifications and combinations of the illustrative embodiments as well as other embodiments of the invention, will be apparent to persons skilled in the art upon reference to the description. It is, therefore, intended that the appended claims encompass any such modifications or embodiments. 

What is claimed is:
 1. A hydraulic jet pump for transference of a fluid medium, the hydraulic jet pump comprising: a housing having a power fluid inlet adapter, a jet nozzle, and a mixing tube positioned therein, the power fluid inlet adapter, the jet nozzle and the mixing tube having a longitudinal axis therethrough; the power fluid inlet adapter communicating through the jet nozzle to the mixing tube along a power fluid inlet flow path; the jet nozzle having a fluid inlet, an area of contraction, and an exit channel, the exit channel having an exit diameter; the mixing tube having a throat transitioning to a diffusing chamber, the diffusing chamber having an outlet opposite the throat; in a first configuration, an axial diverter member traverses the longitudinal axis from the exit channel to the throat, the axial diverter member having an upper end and a lower end, the upper end having a diameter smaller than the exit diameter; in a second configuration, the throat has a first inlet that is axially aligned with the exit channel along the longitudinal axis; and in a third configuration, the throat has the first inlet and a second inlet, the second inlet being angularly offset from the longitudinal axis, wherein one of the first configuration, second configuration, and third configuration is selected prior to downhole deployment of hydraulic jet pump.
 2. The hydraulic jet pump as recited in claim 1, wherein the first configuration further comprises the throat having the first inlet being axially aligned with the exit channel along the longitudinal axis.
 3. The hydraulic jet pump as recited in claim 1, wherein the first configuration further comprises a downhole deployment range of about 0 feet to about 900 feet.
 4. The hydraulic jet pump as recited in claim 1, wherein the second configuration further comprises a downhole deployment range of about 600 feet to about 2700 feet.
 5. The hydraulic jet pump as recited in claim 1, wherein the third configuration further comprises a downhole deployment range of about 2,400 feet to about 12,000 feet.
 6. The hydraulic jet pump as recited in claim 1, wherein the axial diverter member further comprises a gondola shape.
 7. The hydraulic jet pump as recited in claim 1, wherein the axial diverter member further comprises an end with an elliptical nose cone.
 8. The hydraulic jet pump as recited in claim 1, wherein the axial diverter member further comprises an end with a conical shape.
 9. A method for transference of a fluid medium, the method comprising: providing a hydraulic jet pump comprising: a housing having a power fluid inlet adapter, a jet nozzle, and a mixing tube positioned therein, the power fluid inlet adapter, the jet nozzle and the mixing tube having a longitudinal axis therethrough; the power fluid inlet adapter communicating through the jet nozzle to the mixing tube along a power fluid inlet flow path; the jet nozzle having a fluid inlet, an area of contraction, and an exit channel, the exit channel having an exit diameter; the mixing tube having a throat transitioning to a diffusing chamber, the diffusing chamber having an outlet opposite the throat; and selecting one of a first configuration, a second configuration, and a third configuration prior to downhole deployment of hydraulic jet pump, wherein, in a first configuration, an axial diverter member traverses the longitudinal axis from the exit channel to the throat, the axial diverter member having an upper end and a lower end, the upper end having a diameter smaller than the exit diameter, wherein, in a second configuration, the throat has a first inlet that is axially aligned with the exit channel along the longitudinal axis, wherein, in a third configuration, the throat has the first inlet and a second inlet, the second inlet being angularly offset from the longitudinal axis.
 10. The method as recited in claim 9, further comprising selecting the first configuration when a downhole deployment range of the hydraulic pump comprises about 0 feet to about 900 feet.
 11. The method as recited in claim 9, further comprising selecting the second configuration when a downhole deployment range of the hydraulic pump comprises about 600 feet to about 2700 feet.
 12. The method as recited in claim 9, further comprising selecting the third configuration when a downhole deployment range of the hydraulic pump comprises about 2,400 feet to about 12,000 feet.
 13. The method as recited in claim 9, wherein selecting one of the first configuration, the second configuration, and the third configuration further comprises analyzing well head pressure.
 14. The method as recited in claim 9, wherein selecting one of the first configuration, the second configuration, and the third configuration further comprises analyzing target produced flow rate.
 15. The method as recited in claim 9, wherein selecting one of the first configuration, the second configuration, and the third configuration further comprises analyzing maximum velocity.
 16. The method as recited in claim 9, wherein selecting one of the first configuration, the second configuration, and the third configuration further comprises analyzing cavitation pressure.
 17. The method as recited in claim 9, wherein selecting one of the first configuration, the second configuration, and the third configuration further comprises analyzing geometric dimensions.
 18. The method as recited in claim 9, wherein selecting one of the first configuration, the second configuration, and the third configuration further comprises analyzing pump metrics.
 19. The method as recited in claim 9, wherein selecting one of the first configuration, the second configuration, and the third configuration further comprises analyzing power fluid pressure and flow rate.
 20. A method for transference of a fluid medium, the method comprising: providing a hydraulic jet pump having a downhole deployment range, the hydraulic jet pump comprising: a housing having a power fluid inlet adapter, a jet nozzle, and a mixing tube positioned therein, the power fluid inlet adapter, the jet nozzle and the mixing tube having a longitudinal axis therethrough; the power fluid inlet adapter communicating through the jet nozzle to the mixing tube along a power fluid inlet flow path; the jet nozzle having a fluid inlet, an area of contraction, and an exit channel, the exit channel having an exit diameter; the mixing tube having a throat transitioning to a diffusing chamber, the diffusing chamber having an outlet opposite the throat; selecting a first configuration when the downhole deployment range of the hydraulic pump comprises about 0 feet to about 900 feet, wherein, in the first configuration, the throat has a first inlet that is axially aligned with the exit channel along the longitudinal axis and an axial diverter member traverses the longitudinal axis from the exit channel to the throat, the axial diverter member having an upper end and a lower end, the upper end having a diameter smaller than the exit diameter; selecting a second configuration when the downhole deployment range of the hydraulic pump comprises about 600 feet to about 2700 feet, wherein, in the second configuration, the throat has the first inlet; selecting a third configuration when the downhole deployment range of the hydraulic pump comprises about 2,400 feet to about 12,000 feet, wherein, in the third configuration, the throat has the first inlet and a second inlet, the second inlet being angularly offset from the longitudinal axis. 